This project will investigate the interactions of subpopulations of thymus-derived lymphocytes (T cells) in immunity and tolerance. Special emphasis will be given to a subset of naturally proliferating T cells in order to functionally characterize their role in various cell-mediated immune reactions, i.e., graft-versus-host reactions, mixed lymphocyte interactions and responses to T cell mitogens, and to determine the relationship of these cells to classical long-lived non-proliferating T cells. I will also investigate the cellular mechanisms which control both the induction and expression of T cell tolerance in contact hypersensitivity to DNFB and PC1 in mice (suppressor T cells?). These studies will emphasize a characterization of the cells involved and will attempt to elucidate the mechanism(s) of T cell mediated immune regulation.